C6 Rate/extent of chemical change

Question 1

describe collision theory and activation energy

Answer 1

• collision theory: reactant particles must collide with each other
  -> providing particles have sufficient energy to start a reaction
• a collision that produces a reaction is a
  successful collision
• activation energy - minimum amount of
  energy needed for a collision to be successful
• activation energy is diff for different reactions

Question 2

state factors affecting rate of chemical reactions (5)

Answer 2

• temperature
• concentration of reactant
• reacting gas pressure
• SA:V of solid reactant
• presence of a catalyst
  -> all increase rate when increased

Question 3

describe how increased temperature increases rate of reaction

Answer 3

particles gain more energy so move around quicker and collide more frequently

Question 4

describe how increased concentration of reactant increases rate of reaction

Answer 4

more reactant particles in a given volume so more frequent collisions

Question 5

describe how increased reacting gas pressure increases rate of reaction

Answer 5

more reactant particles in a given volume so more frequent collisions

Question 6

describe how increased SA:V of solid reactant increases rate of reaction

Answer 6

-> eg. cutting up the solid into smaller pieces/ into a powder to increase SA
more particles exposed to other reactants so more frequent collisions

Question 7

describe catalysts and how they increase rate of reaction

Answer 7

• substance which increases rate of reaction without undergoing a permanent change
• they provide a different pathway with a lower activation energy
  -> curve showing activation energy on a reaction profile will be lower for catalysed reactions
• not included in the equation
• eg. enzymes used in biological systems

Question 8

state the 2 equations for calculating mean rate of reaction

Answer 8

quantity of reactant used / time taken
OR
quantity of product formed / time taken
-> g/s , cm³/s , mol/s
-> the rate is a mean as the rate varies during the time taken

Question 9

describe reversible reactions

Answer 9

(backwards reaction) ⇌ (forwards reaction)
* direction of reaction changed by changing the conditions - if the reactant is heated, the product is cooled
* if the forwards reaction is exothermic, the backwards reaction is endothermic

Question 10

state Le Chatelier’s principle

Answer 10

if a system is at equilibrium and a change is made to its conditions, the system will respond to counteract the change